When we bootstrap data from excel, the following actions take place:

1. Under Processes, A Timer creates automatically with associated Server action. (To schedule this action with a given an interval of time (Optional))

2. Under data tab, the uploaded excel file saved in resources section and Structure will create with the same excel file name.

Structures are custom type which you can create from inside data tab and it stores the data locally. Here is the clue for your query.

To convert excel data to record List, for temporary purpose we store the Excel data into structures and then converts to record List. You may refer this logic in the corresponding server action from Timer.                          

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